The present invention relates to a distributed control system in which a plurality of built-in systems are connected with a field network such as a CAN (controller area network), a foundation field bus or the like.
A distributed control system in which a plurality of built-in systems are connected with a field network has been applied to a power train control system of a vehicle, a field instrument control system of factory automation or process automation, a control system of a medical instrument or a robot.
In an LON (local operating network) of a kind of field networks or in a standard communication protocol for automobile control OSEK-COM), a library method is employed. In the library method, an application program module (hereinafter, referred to as AP module) calls communication processing. A technology in connection with the library method is described, for example, in “OSEK/VDX Communication Version 2.0a (edited by OSEK group)”.
On the other hand, in most of industrial systems, a memory transcribing method (reflective memory method) is employed. In the memory transcribing method, an AP module and communication processing separated from each other through a common memory are asynchronously operated. A technology in connection with the memory transcribing method is disclosed, for example, in Japanese Patent Application Laid-Open. No 6-332807.
When a message is sent or received through a network, a network controller realizing the network communication by hardware and a network driver performing the network communication using the network controller are used. Here, let a buffer for storing the sent and received message provided in the network controller be called as a mail box. For example, in a case of sending a message, a sending requirement from an application to the network is arranged in a sending requirement queue. The network driver performs network sending processing by taking the sending requirement out of the sending requirement queue one by one and storing the sent message into the mail box. Since the sending requirement queue is generally queued by FIFO, the network sending processing is performed in order of time requiring sending. However, in a real time distributed system, it is desirable that priority is added to the message and the network communication processing is performed in order of the priority. Therefore, as described, for example, in Interface, No. 12, 1994, pages 72 to 148, published by CQ Publisher, there is a method of controlling a network communication in order of priority by changing places of queued sending requirements to order of priority of messages to be sent by an application. Thereby, the network driver can perform network sending processing according to the priority of the sent data at extracting the sending requirements from the sending requirement queue instead of extracting in order of time requiring sending.